1. Field of the Invention
The present invention relates to a photographic exposure apparatus which comprIses an UV-emitting metal-halide discharge lamp for exposing a radiation-sensitive film in contact with a transparent image-bearing sheet.
2. Description of the Prior Art
Known photographic exposure apparatus comprise means for holding a transparent image-bearing sheet in tight contact with a light-sensitive film, a lamp housing wIth an UV-emitting metal halide discharge lamp for carrying out the exposure of the light-sensitive film to the image-bearing sheet, a shutter for closing the lamp housing, forced air cooling means, an adjustable electric power supply for the lamp and control means for controlling the power supply and the shutter, so that the shutter is open and the lamp is operated at full power during the exposure of the light-sensitive film and the shutter is closed and the lamp is kept energized at reduced power during standby of the apparatus, i.e. between successive exposures.
The reason for the operation of the lamp at reduced power during the periods of non-exposure is as follows.
Metal halide discharge lamps have a high radiation efficiency compared with the low-pressure pulsed xenon discharge lamps used on a vast scale for the copyboard lighting of cameras or copyboards. They have, however, one major drawback: they need a running-up time of several minutes and cannot be re-ignited at once without using a high voltage, so that frequent switching is impossible without special measures.
A solution to this problem is to have the lamps permanently burning when the apparatus is in use and to provide the lamp housing with a shutter, which is closed during preparations for the next exposure. In these intervals the lamps are automatically switched to a fraction of full power.
The choice of the power level to which a lamp is energized during its standby position is delicate. On the one hand, this power level should high enough to maintain the bulb of the lamp at a temperature that is high enough to keep the halogen cycle of the lamp going on. On the other hand, this power should be low enough so as not to cause thermal damage to the reflector components of the lamp housing when strongly heated because of the shutter which closes the outlet opening of the lamp housing so that the radiant energy of the lamp remains when within the housing.
Practice has shown that a lamp wattage that still does not cause deterioration of reflector and other parts of the lamp housing, is not really sufficient to maintain a satisfactory halogen cycle in the lamp. This results in a premature blackening of the lamp bulb by deposits of evaporated metal from the electrodes of the lamp. Therefore, the nominal or expected lifetime of the lamp as indicated by the manufacturer is by far not attained.
A typical lifetime of a medium-pressure metal-halide lamp is about 750 h at 5% failure and at a radiation maintenance of 85%. This data is valid, however, for operation of a lamp in almost ideal circumstances, i.e. uninterrupted operation and optimum cooling.
Practice shows that the operational lifetime of this type of lamp in a photographic exposure apparatus under normal working conditions sometimes hardly reaches 400 h. This value stands for the number of hours the exposure apparatus is in use. Since in real terms effective exposure times range only from 5 to 10% of the total period of use, this means that the effective exposure time of a lamp may become as short as 20 to 40 h.